The following relates to electronic ballasts. It finds particular application in conjunction with the high intensity discharge lamps (HID), and will be described with particular reference thereto. However, it is to be appreciated that the following is also amenable to other electronically ballasted lamps such as fluorescent lamps and the like.
A ballast is an electrical device which is used to provide power to a load, such as an electrical discharge lamp, and to regulate its current. The ballast provides high voltage to start a lamp, causing the gas to ionize which begins the process of arc formation. Once the arc is established, the ballast allows the lamp to continue to operate by providing proper controlled current flow to the lamp.
Typically, low frequency, square wave ballasts include a three stage power conversion process. Initially, at stage 1, the AC power line voltage is rectified and filtered. At the intermediate stage 2, the DC voltage is converted to the DC current by a buck converter. At stage 3, the DC current is converted to an AC current by an inverter which includes switches such as MOSFETs to drive the resonant circuit which excites the lamp.
When the lamp is cold, just after ignition, the lamp is characterized by a low impedance. In this condition, the lamp is practically in a short circuit, e.g. the voltage between the lamp terminals is about 20V. Typically, the inverter starts running before the lamp ignites, e.g. the output terminals of the inverter are open prior to ignition. When the lamp ignites, the lamp's impedance quickly drops to about 5% of its stead-state value. As the gas temperature increases in the full arc mode, the lamp voltage increases until it reaches a steady-state voltage. If the output of the inverter is shorted, for example, as the result of a faulty ballast installation process, the inverter MOSFET switches may overheat and also thermally stress the switch of the buck converter of the stage 2.
One approach to prevent the switches from overheating is to use a heatsink. However, such heatsinks are bulky and occupy too much of the ballast space. Other approach is to use one of the thermal management techniques. However, the thermal management techniques are complex and expensive.
The present application contemplates new methods and apparatuses that overcome above referenced problems and others.